1. Field of the Invention
This invention relates to a CRT deflection waveform generator, and more particularly, to a circuit for deriving "S" corrected sawtooth current waveform suitable for driving a CRT deflection yoke in a manner to compensate for geometric distortion inherent in flat faceplate CRTs.
2. Description of the Prior Art
It is well known in the utilization of present day cathode tube display devices that a sawtooth waveform is generally utilized to sweep the electron beam both laterally and vertically across the face-plate. However, as is also known, a uniform electron beam sweeping rate is undesirable because typically the present day CRT faceplate is relatively flat so that the center of curvature around the perimeter of the faceplate is different from that at the center of the faceplate. To correct for this form of built in geometric distortion, a nonlinear correction is introduced into the basic sawtooth deflection current waveform. Creating a signal waveform capable of achieving this desired result has been variously known as geometric distortion correction, pin cushion correction, and "S" correction.
Some prior art circuits which correct for this type of distortion in a CRT have involved devices which modulate a signal on the sawtooth deflection current waveform creating a nonlinear waveform which will cause a deviation in the uniform electron beam sweep rate so that the electron beam will move more slowly at wide angles (near the perimeter of the screen) and more rapidly at narrow angles (near the center of the faceplate).
One specific prior art device for generating an "S" corrected sawtooth waveform is disclosed in U.S. Pat. No. 3,725,726 granted Apr. 3, 1973 to R. F. West, in which a first operational amplifier with a capacitive feedback is used to provide the basic sawtooth waveform generator creating a sweep signal to drive a cathode ray tube yoke. A second operational amplifier also strapped by a capacitor integrates the sawtooth waveform to provide a substantially parabolic waveform which is added at the inverting input of the first operational amplifier. This circuit arrangement requires an active element, i.e., a separate operational amplifier, to integrate the sawtooth waveform creating a parabolic waveform which is then fed back to the input of the basic sawtooth waveform generator thus varying the linear nature of the sawtooth waveform. This system provides a high degree of accuracy but at the expense of the use of multiple active integrators.
Another prior art device for "S" shaping a sawtooth waveform is disclosed in U.S. Pat. No. 3,735,192 granted May 22, 1973 to L. R. Avery. A deflection circuit comprises a pair of capacitors connected in a series relationship to a source of constant charging current. A switching device periodically discharges the capacitors creating a linear sawtooth waveform. Linear control and "S" shaping of the sawtooth waveform is provided by a combination of regenerative and degenerative feedback networks coupled between the output circuit and the junction of the series connected capacitors. The feedback circuits add oppositely phased parabolically varying voltages to the sawtooth waveform to provide the same "S" correction for a linear scanning beam sweep. This device does not develop a compensating waveform which corrects the basic sawtooth waveform with a high degree of accuracy.
Of general interest in the generation of a linear sawtooth waveform suitable for use in the vertical deflection circuit of a CRT are the devices disclosed in U.S. Pat. No. 3,890,542 granted June 17, 1975 to G. Zimmerman and U.S. Pat. No. 3,784,857 granted Jan. 8, 1974 to T. J. Christopher. Both of the circuit configurations disclosed in these patents utilize an operational amplifier in which a capacitor is strapped between the inverting input and the output of the amplifier providing a negative feedback signal proportional to the current through the deflection coil. A switch shunting the capacitor is rendered conductive during the retrace interval discharging the integrating capacitor and creating a high voltage across the deflection yoke which retraces the electron beam.